When an engine is warmed up, it is preferable that a temperature of lubricant oil for the engine is increased quickly so as to reduce a friction loss between a cylinder block portion and pistons of the engine, for example. In this case, a temperature (cylinder block temperature) of the cylinder block portion may be increased in preference to increasing a temperature (cylinder head temperature) of a cylinder head portion of the engine having a combustion chamber. Accordingly, the friction loss can be reduced effectively.
In a circulation system described in Patent Document 1 (JP 6-193443 A), a cylinder block portion of an engine has a cylinder block passage through which a coolant flows, and a cylinder head portion of the engine has a cylinder head passage though which the coolant flows. The cylinder block passage and the cylinder head passage are connected in parallel. In a warm-up operation of the engine, a coolant temperature (cylinder block temperature) in the cylinder block portion is increased more rapidly than a coolant temperature (cylinder head temperature) in the cylinder head portion is, by reducing an open degree of a control valve that controls a flow rate (cylinder-block flow rate) of the coolant flowing thorough the cylinder block portion.
Recently, an engine is equipped with an exhaust gas recirculation system (EGR system) in which a part of exhaust gas adapted as EGR gas flows back to an intake side of the engine, and the EGR gas is cooled by an EGR cooler. The EGR cooler, i.e., a heat exchanger is provided in an EGR pipe that connects an intake pipe and an exhaust pipe, and the EGR cooler exchanges heat between the EGR gas and a coolant. Here, a coolant used for the engine is generally distributed to the EGR cooler, and in this case, it is preferable that the coolant is distributed to the EGR cooler at a flow rate optimized for heat exchange in the EGR cooler.
In addition to the EGR cooler, there are various heat exchangers and portions to which coolant is required to be distributed. The various heat exchangers and portions include a coolant passage (heat exchanger) provided in an EGR valve that controls a flow rate of the EGR gas, a coolant passage (heat exchanger) provided in a throttle valve that adjusts an intake air amount, an oil warmer (heat exchanger) that heats lubricant oil, and a heater core (heat exchanger) that heats conditioned air. When the coolant is distributed to these heat exchangers, a coolant flow rate distributed to each of the heat exchangers is preferably adjusted at a coolant flow rate desired for heat exchange performed in each of them.
However, the above-described conventional circulation system does not have a device that controls a coolant flow rate (cylinder-head flow rate) in the cylinder head passage. Thus, the cylinder-head flow rate is increased in accordance with decrease of the open degree of the control valve that controls the cylinder-block flow rate. Therefore, the cylinder-block flow rate is adjustable independently, but the cylinder-head flow rate may be unadjustable independently. When the coolant is distributed to the above-described various heat exchangers from the cylinder head passage in the conventional circulation system, the coolant may not be distributed to the heat exchangers at the desired flow rates.
Also when the coolant is distributed to the various heat exchangers from the cylinder block passage, the coolant may not be distributed to the heat exchangers at the desired flow rates in a case where the cylinder-block flow rate is reduced during the engine warm-up operation.